Correlation of Alteration in Chromosome Replication Sequence with Cell Lethality

This study is a comparison of two methods of analyzing DNA replication in bacteria. Oxygen sensitization of DNA replication to X-irradiation was studied by (1) analysis of the replication sequence (qualitative measurement) and (2) assay of the amount of incorporation of radioactive precursor into the DNA fraction (quantitative analysis). An alteration in the sequential replication of the bacterial chromosome was observed within 10 minutes of postirradiation incubation at a colony survival of 65 to 75%. A 7500-R dose of X-rays with nitrogen bubbling induced the same extent of alteration in replication sequence as a 2000-R dose with oxygen bubbling, resulting in a dose-reduction factor of 3.75. As a result of radiation doses allowing 70% survival, a clear and immediate alteration of the replication sequence was evident. In contrast, only a slight delayed effect on DNA synthesis was reflected when the rate of radioactive thymine uptake was measured. These results indicate that analysis of sequential DNA replication is a relatively sensitive and reliable index of lethal chromosome injury, which is correlated with lethal cell injury.